Researchers can detect impact of electrons shared between atoms.

The covalent bonds that hold complex molecules together can come in different forms. Atoms like carbon, nitrogen, and oxygen can form both single and double bonds, sharing two or four electrons. Nitrogen and carbon can even form a triple bond, sharing six. And those are some of the simpler ones. A mixed series of single and double bonds, like those found in benzene, can end up creating a diffuse electron cloud, so that each of the bonds has an odd number of electrons.

The exact strength of these bonds depends strongly on the context of the surrounding molecule. It's possible to get a variety of information about these bonds. We could calculate what their energy is, probe them with chemical reactions, and could even detect the difference in bond strength by imaging the structure of a crystalized population of molecules. But now, a consortium of researchers in Europe have figured out how to use a modified form of atomic force microscopy to examine the strength of chemical bonds in a single molecule.

The rules of covalent bonds are, at least on the surface, quite simple. The more electrons that are shared, the stronger the bond. And the stronger the bond, the closer together the two atoms on either end of it will be. As we said above, though, things get complicated when there is a mixture of single and double bonds, which can create a set of delocalized electrons. The simplest form of this is a benzene ring, which has six carbon atoms arranged in a circle, linked by three single and three double bonds. In this case, the bonds all become equivalent, and each atom is linked by what you could consider 1.5 bonds, instead of a single or double.

Things get even more complex when a benzene ring is embedded in a larger molecule, with other single and double bonds surrounding it. Take a buckyball, in which carbon atoms are linked in a set of interconnected five- and six-atom rings. The six atom rings have benzene-style alternating bonds, while the five atom ones are all linked through single bonds. So a given atom may be part of two benzene rings and a pentagon, all at the same time.

Instead of nice, clean 1.5 bonds, these arrangements create fractional shared electrons, leading to very small differences in energy and distance between adjacent atoms.

All of which makes detecting the difference that much more of an achievement. Standard atomic force microscopy relies on a needle with a single atom as a tip, and that's able to probe the electronic conditions in a molecule or surface. But it doesn't have enough resolution to detect differences in bond length. Instead, the team behind the new paper used a modified form, in which that single atom is capped by a carbon monoxide molecule, meaning the tip of the needle juts out by two extra atoms.

This turned out to be critical. In a number of the samples, the difference in bond length is expected to be at or below the best resolution of atomic force microscopy. As the tip gets pressed down, the carbon monoxide molecule can flex out of the way, behavior that appears to amplify the length of the bond. As a result, the authors were able to discriminate down to bonds that differed by only 0.03 Angstroms (3.0 × 10-12 meters).

The team started by imaging a buckyball, where they could see the difference in bonds between those in five and six membered rings. They then moved on to more complex molecules, such as the one shown on top.

Aside from being an impressive technical achievement, the technique should open up a number of potential opportunities. Molecules that don't form crystals easily could be imaged with this technique, and the extremely precise control could allow researchers to inject electrons or probe chemical conditions at specific points in a single molecule. All of which could help provide a better understanding of the reactions and catalysts that we rely on for many of the basic materials we use every day.

This turned out to be critical. In a number of the samples, the difference in bond length is expected to be at or below the best resolution of atomic force microscopy. As the tip gets pressed down, the carbon monoxide molecule can flex out of the way, behavior that appears to amplify the length of the bond.

I'm having a little trouble visualizing this. The CO molecule bends away from the bond being probed, and that in turn "stretches" (for lack of a better immediate term) that bond to a size detectable by the traditional single Au tip? And the stretch factor is a function of the original bond length?

Isn't it really the case that in benzene, all of the bonds are the same? It's often drawn as if there were alternating single and double bonds, so the number of carbon bonds adds up to the expected four, but from an electron probability distribution point of view, if I understand correctly, the whole ring has the same probability to find an electron.

Isn't it really the case that in benzene, all of the bonds are the same? It's often drawn as if there were alternating single and double bonds, so the number of carbon bonds adds up to the expected four, but from an electron probability distribution point of view, if I understand correctly, the whole ring has the same probability to find an electron.

Yes, the article says "In this case, the bonds all become equivalent, and each atom is linked by what you could consider 1.5 bonds, instead of a single or double."

Standard atomic force microscopy relies on a needle with a single atom as a tip

We can create needles with a tip of a single atom?

And yet we don't have sexbots.

Mankind took a wrong turn somewhere.

I've used standard AFM before, and I seem to remember the tips being extremely delicate and expensive. Cool article

Hah, I know there's a joke in there somewhere.

On topic, I would guess this would also improve precision for single molecule studies. I remember a paper where they looked at controlling a chemical reaction between two molecules with the use of an AFM (it was a while ago, maybe this is more commonplace now?).

Standard atomic force microscopy relies on a needle with a single atom as a tip

We can create needles with a tip of a single atom?

And yet we don't have sexbots.

Mankind took a wrong turn somewhere.

I've used standard AFM before, and I seem to remember the tips being extremely delicate and expensive. Cool article

Hah, I know there's a joke in there somewhere.

On topic, I would guess this would also improve precision for single molecule studies. I remember a paper where they looked at controlling a chemical reaction between two molecules with the use of an AFM (it was a while ago, maybe this is more commonplace now?).

I don't think 3 picomoles per litre is the same as 0.03 Ångstroms, but you could say 3 pm. Then again, Å are a fairly common unit when talking about bond lengths in chemistry, because they tend to be in the 1-3 Å region.

I don't think 3 picomoles per litre is the same as 0.03 Ångstroms, but you could say 3 pm. Then again, Å are a fairly common unit when talking about bond lengths in chemistry....

... because chemists are not right in the head.

But what I am wondering is what is the meaning of "bond length"? Does it mean internuclear distance? Surely A standard AFM should get you that anyway. It though the headline "bond strength" made more sense, even though I still don't understand the measurement.

It's not just because chemists are weird, it is because crystallographers and many physicists are weird too. Using angstroms is exceptionally convenient because atom sizes and bond lengths all tend to be in the neighborhood of 1-4. The other answer is "for historical reasons", but of course our predecessors noticed the convenience. If the standard were the Bohr radius instead (a reasonable "natural unit" at this scale) then I might manage to get a bit worked up about it.

adrian.ratnapala wrote:

But what I am wondering is what is the meaning of "bond length"? Does it mean internuclear distance? Surely A standard AFM should get you that anyway. It though the headline "bond strength" made more sense, even though I still don't understand the measurement.

I'm more of a powder diffraction guy, not an AFM guy, but one could think of bond length like "mean distance between nuclear centers" for most purposes. Often that isn't directly probed because interactions in many experiments are with the electrons, so in practice the measured bond length will be a reflection of the electron distribution/density. In x-ray diffraction, for example, the scattering angle is directly related to the distance between the scattering electrons, but it can be convenient to think of that as the distance between the atoms which contain those electrons.

Naturally, one must move away from that simple picture once the effects of interest have size similar to what the simplification ignores. In this case the resolution of the AFM determines how precisely the bond length can be measured. If the spatial resolution of the measurement is worse than the difference between two different types of bond lengths, for example, then the measurement cannot actually distinguish the difference in length.

Standard atomic force microscopy relies on a needle with a single atom as a tip

We can create needles with a tip of a single atom?

And yet we don't have sexbots.

Mankind took a wrong turn somewhere.

Not always and not for all AFM needles. If I remember it correctly it is akin to the statistical processing of ICs, just not always parallel products. There are procedures that make them useful - or not.

A huge motivation behind AFMs is that they lead up to understanding microscales (plus they are cheap and fun), which will lead to better understanding of electronics, chemistry and biology, which will lead up to sexbots. It is always about the sexbots.

Is it only US citizens that write M instead of m for meters? I also see a lot of MM which is in principle even worse: you no longer know if they mean mm (millimeters) or Mm (megameters), though odds are for the former of course.

I don't think 3 picomoles per litre is the same as 0.03 Ångstroms, but you could say 3 pm. Then again, Å are a fairly common unit when talking about bond lengths in chemistry....

... because chemists are not right in the head.

But what I am wondering is what is the meaning of "bond length"? Does it mean internuclear distance? Surely A standard AFM should get you that anyway. It though the headline "bond strength" made more sense, even though I still don't understand the measurement.

Technically, it should mean the separation between nuclear centres. And yes, a conventional AFM gets you that number. But its resolution limits your ability to distinguish between bonds that are almost, but not quite exactly, the same. So, if the resolution was 0.1 Å, bonds that are 1.53 and 1.51 Å would appear to be the same.

Actually, I was just teaching about resolution yesterday - this would have been a good example to use in class!